This invention relates to an electric hotplate with a thermostat, in which the heating plate is joined to the surrounding cold or unheated zone by means of a bridge of relatively thin sheet metal.
U.S. Pat. No. 3,909,592 discloses a hotplate which consists of a continuous plate of metal, for example stainless steel, with one or several hot and cold zones brought about by the plate's being whole and continuous on the top side, but with the underside divided by unbroken grooves which surround the hot zones. The thin layer of stainless steel bridging the cold and hot zones will act as a circular hinge and convert the horizontal heat expansion of the heated zone to a vertical movement, resulting in a parallel displacement, in the same plane, of the heated zone. The present invention is based on this earlier design and exploits, in an advantageous manner, the possibilities which this has opened for achieving a simple, inexpensive and robust solution to the thermostatic control of electric hotplates.
Controlling the temperature of electric hotplates by means of thermostats or energy regulators is known in many different forms, which to a greater or lesser extent have certain failings and inadequacies.
Bi-metal thermostats are very sluggish, and this is a problem attached to all known mechanical systems for controlling the temperature of electric hotplates. This sluggishness or inertia can vary somewhat, but for most thermostatically controlled hotplates the difference between cut-in and cut-out will be around 30.degree. - 50.degree. C. depending upon the type of thermostat, the positioning of the thermostat and the mass of the hotplate itself.
Electronic temperature control by means of thermistors or thermocouples is also known. It is further theoretically possible to build an electronically controlled system with a large number of thermocouples or thermistor probes distributed over the whole hotplate, but such a system would, on account of the cost, be of little practical interest. A further weakness of such a system would be that with the large number of components involved, the risk of failure would be correspondingly high.
Probably the most relevant known technique in connection with the present invention will be found in Norwegian Patent No. 122,668 which discloses the exploitation of the hotplate's horizontal heat expansion to operate a thermostatic switch. The movement of the hotplate is transmitted to the thermostatic switch by means of a rod, the ends of which are fixed to the underside of the plate, the rod extending across the plate. Contrary to the principle on which the present invention is based, this known construction is not based on the fact that the heating plate itself moves vertically as a function of its temperature.
For further elucidation of present art, reference is also made to U.S. Pat. No. 2,898,439 which describes a thermostat which can be used in connection with a hotplate. The upper part of this thermostat extends over the hotplate, and is pressed down to operate contacts when a saucepan or the like is placed on the hotplate. In other words, there is disposed a separate temperature probe for the thermostat.